[AMRadio] DX-100B on Phone - bad audio distortion / oscillation

[ne1s at securespeed.us]

Rick and list, see comments below.

> Connected up my off-center fed dipole for 80 and 40, which I have
> used for a couple of years on a few rice boxes and a Collins KWM-2A
> and it always worked acceptably if not spectacularly.  Tuned it up on
> 3885 and listened for a while, putting in an occasional call.  Seemed
> to work OK for a while but then the output on the scope just took off
> into some kind of oscillation.  I backed off the audio gain and the
> drive to zero and no joy, had to switch from Phone to CW to make the
> oscillation go away.
>
> Since then, it has been kind of random... sometimes it works fine and
> gets good audio reports on 3885, other times it takes off into
> self-oscillation.
>
> The transmitter is rock-stable in CW, this only happens when I switch
> to Phone, and then not all the time.
>

These are strange symptoms. I would otherwise think it is RF rectification
into the audio, except  you say you can turn the AF gain all the way down
and the oscillation persists. It's even weirder that you can turn the
drive pot all the way down and the oscillation persists.

Does the RF output frequency change when it goes into oscillation, and you
kill the RF drive? I'm thinking perhaps the finals are taking off on their
own, since they appear to be generating RF with no drive from the 5763. If
so, you may need to add neutralization. But I can't explain why this would
happen in AM and not CW.

Another possibility is an ultrasonic oscillation in the modulators, which
may be because you've got some really "hot" 1625s. But this wouldn't
explain why reducing drive to zilch doesn't kill the RF output.

Sorry, so far I can't come up with anything that fits all the symptoms you
describe. Maybe I'm not interpreting something correctly. If I think of
something later, I'll post it.

Good Luck, Rick!

73,
-Larry/NE1S